North Korea says it has successfully tested a hydrogen bomb amid reports of a tremor near the main nuclear test site.
State media announced the test after monitors detected a 5.1 magnitude quake close to the Punggye-ri site.The North is thought to have conducted three previous underground nuclear tests there since 2006.
A hydrogen bomb uses fusion to create a blast far more powerful than that of a more basic atomic bomb.
Analysis: Stephen Evans, BBC Korea correspondent
North Korean leader Kim Jong-un was shown attending an apparent submarine missile test last year
This test was apparently a hydrogen bomb, a step up in destructive power from the plutonium used in previous tests. It gives more explosive power for a lighter weight.
After the test in 2013, there was widespread condemnation. The UN Security Council held an emergency meeting at which its members, including China, "strongly condemned" the test. Similar outrage is expected this time. Prime Minister Abe of Japan has said this fourth test was a "serious threat to the safety of his nation".
On top of any fourth nuclear test, North Korea also appears to have tested a submarine-launched missile. The ability to launch missiles from submarines would change the whole calculation of military response because warning times of an attack on, for example, the West Coast of the United States would be much shorter.
Before the test, North Korean state media said the country "deserved to hold nuclear weapons... to counter nuclear threats by the US".
Experts believed before the fourth test that North Korea was still some years from being able to hit a target with a nuclear bomb delivered by a missile. But it is crystal clear that it is absolutely determined to be able to do so. It is also clear that it is improving its abilities rapidly.
If confirmed, it would mean Pyongyang is intent on pursuing its nuclear programme with little regard for the major political and diplomatic costs that will inevitably accompany this unwelcome development, says Dr John Nilsson-Wright of Asia Programme at Chatham House.
In a surprise announcement a newsreader on North Korean state TV said: "The republic's first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016."
Last month, North Korean leader Kim Jong-un said Pyongyang had developed a hydrogen bomb, although international experts were sceptical.
What is a hydrogen bomb?
- A weapon energised by the nuclear fusion of hydrogen isotopes in a chain reaction, developed in 1958 by the United States
- Also known as a thermo-nuclear bomb, it is seen as a "cleaner" bomb than an atomic one as it has less radioactive fallout - but also much more powerful
- Unlike an atomic bomb, powered by nuclear fission, a hydrogen bomb is powered by the fusion of lighter elements into heavier elements
- Such bombs can be as small as a few feet long and can fit in warheads of ballistic missiles
Suspicion of a test was first raised after the US Geological Survey said the epicentre of the quake - detected at 10:00 Pyongyang time (01:30 GMT) was in the north-east of the country, some 50km (30 miles) from Kilju city, near Punggye-ri.
The BBC's Kevin Kim in Seoul says analysts will now focus on trying to detect if any gases have leaked from the subterranean explosion to conclude what type of nuclear material may have been used, if it indeed it was a test of a hydrogen bomb.
North Korea and nuclear weapons
October 2006: The first of three underground nuclear explosions is announced, at a test site called Punggye-ri
May 2009: A month after walking out of international talks on its nuclear programme, North Korea carries out its second underground nuclear test
February 2013: A third nuclear test takes place using what state media calls a "miniaturised and lighter nuclear device"
May 2015: Pyongyang claims to have tested a submarine-launched missile, which are more difficult to detect than conventional devices
January 2016: North Korea says it has successfully tested a hydrogen bomb
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